Neutron detection is used for a variety of purposes. For example, neutron detectors are used to enhance safety in nuclear power facilities because neutron radiation can be a safety hazard with use of nuclear reactors. With the development of nuclear weapons, there has been an increasing need for high sensitivity neutron detectors to safeguard nuclear materials and weapons, to verify treaty and regulations compliance, and to recover military payloads. Significantly, neutron detectors are needed to minimize the risk of nuclear weapons proliferation. Many are concerned that weapons grade plutonium or other radioactive materials may be stolen and transferred across country borders for use by terrorists or warring factions or countries. Neutron detectors may be used at ports of entry such as harbors, border crossings, and airports to detect the presence of radioactive materials such as plutonium that release neutrons as neutrons cannot be easily hidden with shielding. Such neutron surveillance must be accomplished without undue restriction or disruption of traffic flow and events.
Unfortunately, neutron detection is not an easy science, and developers of neutron detectors face a number of difficult challenges. A naturally occurring neutron fluence is always present, and this fluence varies with the molecular composition of adjacent soil, water, buildings, and so on as well as with latitude and elevation. The time variance of the background fluence has been described as having a Poisson distribution with respect to time. Thus, the extraction of meaningful data has to rely on obtaining sufficient data to make statistically meaningful conclusions. Typically, when searching for contraband neutron sources, the neutron flux emission is very low and not readily separated from the background signature; thus, large detectors can collect data more rapidly than smaller ones. Another challenge in designing a neutron detector is that neutrons are electrically neutral, do not respond to electric fields, and are weakly interacting with electrons. Hence, neutrons do not ionize atoms except by direct collision with nuclei of few selected element isotopes, which makes conventional gaseous ionization detectors ineffective in neutron detection.
Presently, costly and bulky pressurized tubes using rare Helium-3 gas are used to detect neutrons. These conventional neutron detectors are considered to be within a class of conventional neutron detectors labeled gas-filled counters that require the application of high voltage and gamma rejection circuitry. In practice, the Helium-3 filled tubes also require careful handling since they can indicate false positives when abruptly moved or struck (e.g., provide an undesirable microphonic response). These types of conventional neutron detectors are effective in many types of field operations, but they are not suitable for operations requiring compact (e.g., covert) and highly sensitive devices capable of functioning for long periods of time with low power consumption. In addition, these types of detectors are typically hand-fabricated and use Helium-3 gas that is generated in a nuclear reactor, making them expensive to manufacture in any quantity. The high cost of these devices has severely limited their deployment in areas such as border crossings, cargo container inspection equipment, and the like where they could be used to detect movement of contraband such as plutonium or plutonium-based weapons.
In some attempts to create an improved neutron detector, some researchers have used solid-state electronics to sense alpha particles emitted from a neutron converter material in which a reaction has taken place in which a neutron has collided and generated one or more alpha particles. The role of the converter material is to convert incident neutrons into emitted charged particles, which are more readily sensed. When the emitted charged particle transits a semiconductor device, it liberates bound charges in its wake, and these charges may be collected and used to sense the event stimulated by the initial neutron reaction. Such devices therefore serve as neutron detectors including converter material and a semiconductor-based detector. For example, a boron-10 and lithium-6 metal, e.g., a neutron detection layer, has been applied directly to a crystalline device (such as a gallium arsenide crystalline PIN diode) to provide a neutron detector. However, the use of crystalline diode structures in neutron detectors has its own set of drawbacks and limitations. The internal noise level of an uncooled crystalline diode is appreciable, and consequently researchers have found it difficult to measure low background levels of ambient thermal neutrons in the surrounding area or to detect single neutron events using diodes of any consequential size. A typical crystalline diode also has a thick semiconductor layer in which charges are collected, and it can be expensive and difficult to grow large crystalline detectors. Charges liberated by gamma rays are also collected in the thick semiconductor layer, and these charges contribute to the non-neutron noise signal of the detector. It is imperative that gammas not be mistaken for neutrons since in a typical environment the background gamma fluence greatly exceeds the neutron fluence expected from a typical source of neutrons.
The drawbacks associated with such solid-state neutron detectors including high cost, small size, and difficulty to manufacture have resulted in continued use of the bulky and expensive Helium-3 pressurized tube devices to detect neutrons. There remains a need for a high sensitivity neutron detector that can be more easily manufactured, that is less expensive (e.g., allowing neutron detectors to be more widely implemented and used), and that can be readily scaled in size (e.g., monitor a larger surface area to support detection of radioactive materials such as smuggled plutonium and plutonium-based weapons hidden in moving objects such as objects on conveyor belts and the like).
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.